The present invention relates to improvements in the field of borehole expansion testing. More particularly, the invention is concerned with an improved borehole dilatometer for determining the deformability of rock masses.
In general, laboratory tests on small rock specimens are inadequate to measure rock mass deformability because of scale effects. This limitation has led to the development of several static and dynamic field methods. In the static methods, loads are applied on selected rock surfaces in boreholes, underground galleries, or against surface exposures, and the resulting deformations are measured. These methods include plate bearing tests, flat jack tests, gallery tests and borehole expansion tests. Compared to large scale field tests, such as gallery and plate loading tests that are expensive and time consuming, borehole expansion tests can be conducted within a much more reasonable cost and time frame. However, they tend to involve smaller rock mass volumes.
A number of borehole testing devices are presently available which apply a load and measure the direct response of the borehole wall. These devices either (a) supply a uniform internal pressure to the borehole wall, such as dilatometers, or (b) supply a unidirectional pressure to a portion of the borehole wall by forcing apart two diametrically opposed curved steel platens, such as the NX-borehole jack described by Goodman, R. E. et al, in Proc. 10th US Symp. on Rock Mechanics, pp. 523-555, 1972, and by Heuze, F. E. et al, in Int. J. Rock Mech. Min. Sci., Vol. 22, No. 2, pp. 105-112, 1985. The results of these tests are often presented in the form of curves showing applied pressure versus diametral deformation. The diametral deformation is measured either in the direction of loading, as with the borehole jack, or at several locations around the borehole circumference as with the dilatometer described in Canadian Patent N.degree.840,203.
Usually, borehole expansion test results are analyzed by modeling the rock mass as a linearly elastic, isotropic and homogeneous continuum with Young's modulus and Poisson's ratio. A value for the Poisson's ratio is assumed (often equal to 0.25) and the rock mass deformation modulus is determined from measured values of applied pressure and hole diametral deformation through equations borrowed from the theory of linear elasticity for isotropic media.
Existing dilatometers and the NX-borehole jack have several limitations. Dilatometers can only apply a uniform pressure along the wall of a borehole and are not directional, i.e., they cannot load the rock in a predetermined direction. Unless multiple transducers are installed in the instrument, any variations in diameter change around the borehole circumference due to inherent rock mass anisotropy, discontinuities and heterogeneities cannot be recorded. Many dilatometers still measure a volume change which is analyzed in terms of rock mass deformation by assuming that the rock is isotropic. On the other hand, the borehole jack was designed to apply directional loading in boreholes. However, the major flaw with the borehole jack is that the contact between the rock and the curved steel platens is not uniform and cannot be determined exactly, like for the dilatometers. The contact angle varies with the applied load and the difference in deformability between the steel and the rock. Since the contact angle enters into the calculation of the rock modulus of deformation, this may result in some errors when determining rock mass deformability.